While working on a font, a good tip is to leave the Font Test Window open, then switch to WordPad or Word to test it. The temporary font created by the Font Test will be named “FCP 012345” or similar.
Otherwise, you need to uninstall, save your font, and reinstall every time you add one more character. Don’t worry, the program works just fine. Read some of the Frequently Asked Qestions at the top of the support forum.